The process of drilling bore holes involves in part the circulation of drilling fluid from a series of surface storage tanks down the inside of the drill string, through the drill bit, and up the annular space between the drill string and the bore hole back to the surface storage tanks.
Several functions are performed by the drilling fluid during circulation:
1. Cool the drill bit. PA1 2. Transmit hydraulic power to the bit from engines at the surface. PA1 3. Remove cuttings from around the drill bit and transport them to the surface. PA1 4. Maintain the chemical and physical integrity of the wall of the well bore. PA1 5. Prevent the influx of formation fluids and gases into the bore hole. PA1 6. Release drill cuttings to waste by settling and by the action of various mechanical systems normally provided to a drilling rig (screens, hydrocyclones and centrifuges).
It is generally known that drilling efficiency which is partly determined by the rate of penetration and bore hole condition can be increased by proper selection of, or improvements to, the drilling fluid.
A lower viscosity drilling fluid as compared to a higher viscosity drilling fluid can be instrumental in providing a larger proportion of surface hydraulic power to the drill but because the pressure losses through the pumps, connections and interior of the drill string are minimized. Penetration rate is partly related to the hydraulic power supplied to the drill bit.
A lower viscosity drilling fluid also contributes to higher penetration rates because of higher efficiency in the formation and removal of cuttings from below the drill bit during the instant in which the drill bit crushes the rock.
A low viscosity drilling fluid as compared to a higher viscosity drilling fluid also ultimately promotes drilling efficiency by more effectively releasing drill cuttings in the various surface solids removal processes.
Knowledge of the drilling process indicates that penetration rate is in part related to the density of the drilling fluid or more explicitly the differential pressure across the interface between the drilling fluid and the uncut formation rock below the drill bit. The lower the differential pressure (pressure due to the drilling fluid minus the pressure due to the formation) the higher the penetration rate.
Bore hole integrity is one of the major contributors to drilling efficiency in that many problems such as drill string torque and drag, stuck drill string, loss of drilling fluid circulation, and poor bore hole cleaning can be avoided or diminished by good bore hole conditions. It is known that many well bore integrity problems are associated with the negative effects of water on various geological formations that may be penetrated while drilling. To avoid these effects, various types of oil continuous phase drilling fluids have been devised and used.
By far the best known and most widely used oil continuous phase drilling fluid is an invert oil emulsion which incorporates an aqueous salt solution in the form of very fine droplets as an internal or discontinuous phase, this internal or discontinuous phase usually constitutes 3% to 30% of the liquid portion of the drilling fluid. This invert oil emulsion drilling fluid is stabilized by "water in oil" emulsifiers, surfactants, and oil wetting agents. This invert oil emulsion drilling fluid can be viscosified with chemically treated clays or in some cases, polymeric viscosifiers.
It is known, and generally understood, that when water is emulsified in oil, as an internal or discontinuous phase, the viscosity of that oil increases as the percentage of water emulsified increases.
In the context of invert oil emulsion drilling fluids, bore hole integrity is related to the concentration and type of salt water solution used to produce the internal or discontinuous phase and each geological formation that may be drilled can theoretically require a somewhat different salt water internal or discontinuous phase to assure bore hole integrity. Invert oil emulsion drilling fluids are formulated to provide salt water internal or discontinuous phases which tend to dehydrate through the process of osmosis in most geological formations and thereby minimize well bore hole damage. Although this approach has been successful in producing stable bore holes it is nevertheless a compromise and constitutes good reason to contemplate and search for an oil continuous phase drilling fluid which could function without a salt water internal or discontinuous phase.
It has been a practice to drill with an oil drilling fluid without any added materials at least initially. As drilling progresses, however, the oil drilling fluid accumulates water from the surface (rain, snow, spills etc.) and/or from water generated by the drilling of certain geological formations. This accumulation of water tends to lead to a variety of drilling problems including the formation of mud rings (clumps of drill cuttings sticking to one another) the blinding of drill solids removal screens caused presumably by the variable wetting (oil and water) of the screen, and the loss of bore hole integrity through the absorption of this free water by sensitive geological formations. The normal response to this situation is to add, one of, or a combination of, water in oil emulsifiers, surfactants and wetting agents to the drilling fluid. This has the effect of oil wetting the drilled solids and drilled solids removal screens and emulsifying excess water so that it becomes an internal or discontinuous phase. The net result is the transformation of what began as a water free oil drilling fluid to inverted oil emulsion drilling fluid.
The principle object of the present invention is to avoid or at least delay the changing of a water free oil drilling fluid to an invert oil emulsion drilling fluid because of the presence of water. A further object of the invention is to maintain a water free oil drilling fluid without changing or minimally changing current drilling and drilling fluid practices.